I have been painting my house, and admittedly, I have not been using primer. The baseboards, however, are a darker color, and painting them white has taken several coats (somespots up to 4 coats) to get the old color to cover. I have gone through a lot of white paint because of this.

My question is, if I use primer now on the baseboards, will the tint be off because of the primer underneath as opposed to just the original paint color and paint? I'd hate to have to reprime and paint all the baseboards I have done up to this point, but if priming them first and then repainting them is what needs to be done, then I will do so. If I can get away with just priming what's left, and painting the white over the primer without a noticeable tint difference, then I will do that.

Also, what is the best color primer to use with white? I was thinking like that light grey primer, but I am not sure the white would cover that very well. I was also thinking white primer, but I think it would be hard to tell the difference between what's painted already and what's only primered (even more so on second coats).

2 Answers
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I rarely use primer on previously-painted walls unless the existing finish is suspect with regard to bonding. It's a disaster to have paint come loose due to incompatibility.

The right primer color when doing dramatic color change is usually whatever gets you close to your final color. You want whatever underlies your topcoat to have minimal effect on the color you selected. That said, sometimes a base coat can affect the topcoat in desirable ways, such as making the color deeper or more rich. That usually applies to bold colors, though, and gray (or custom-tinted) primer is often appropriate.

You ask about changing approaches mid-stream. That is a potential concern, but you'll have to try it and see. If the primer approximates what the first coat of paint did, it may not be noticeable.

When painting white over white primer, it's usually not as difficult as you might think to see what you're doing. Wet paint has a gloss, dry paint usually has more sheen than primer, and the color is rarely that close anyway. Use good lighting, good technique, and it shouldn't be a problem.

Thank you for your response! The first coat I put on over the existing paint didn't do very much. I would imagine the primer, since it's a thicker and heavier base, would probably cover up a lot more. I just didn't think it would take 4 coats of white (more in some areas) to cover up the existing color. It's darker than white, but it's still a lighter color (Its like a weird looking tannish/brown color).
– Ex0rNov 9 '17 at 19:07

Purpose of prep

The main issue with surface prep is to scuff-sand so you are removing any gloss/sheen and giving the new paint a microscopically rough surface to sink its teeth into. A 3M scotchlite pad (e.g. Green) is plenty for this, and I like them because it's not so easy to sand through all the paint on corners and ridges.

Then, you do a washdown to remove dust, oils and other contaminants. If you're in a hurry, you use the same solvent your paint uses for thinner, that way you can paint straightaway without worrying about unevaporated wipedown solvent contaminating the paint.

What primer does

Primer has several purposes: but the main one in interior work is to render the surface entirely uniform and nonporous, so the new paint will apply consistently, instead of mark through, giving different surface sheen for different surfaces.

It is also a preservative, aiming to stop rot and rust. It also provides a barrier coat to try to arrest chemical incompatibility with previous coats, though after many years chemical incompatibility fades away.

On already painted surfaces, primer is much less important than surface prep. If you paint a contaminated or glossy surface, that paint will fall off in a few years. You've seen what that looks like.

Covering that color

One thing primer can be very good for is cover-up. Some primers have strong pigments to help cover dissimilar colors, and can be much better at it than paint. I was going to try to no-primer an interior job, til I realized I'd need 2 coats to fully hide the disgusting tan off-white... So I made one coat primer and solved all surface issues in doing so. Primer is cheaper anyway.

Definitely use white primer under white paint, unlesss the gray primer is much better at covering up! If you use non-white, the paint will have to fight much harder to cover correctly.

Also cheap paint has weak pigments, which require more coats to cover. White pigment is easy, it's a reliable mineral pigment such as titanium dioxide. Cheap paints cheap out on quality or coverage.

You should put enough layers on to fully coverup the last paint. That can be a chore.